As is well known to those having skill in the art, monocrystalline silicon carbide is particularly well suited for use in semiconductor devices, such as integrated circuit semiconductor devices and power semiconductor devices. Integrated circuit semiconductor devices typically include many active devices such as transistors in a single semiconductor substrate. Power semiconductor devices, which may be integrated circuit devices, are semiconductor devices which carry large currents and support high voltages.
Silicon carbide has a wide bandgap, a high melting point, a low dielectric constant, a high breakdown field strength, a high thermal conductivity and a high saturated electron drift velocity compared to silicon, which is the most commonly used semiconductor material. These characteristics allow silicon carbide microelectronic devices to operate at higher temperatures and higher power levels than conventional silicon based devices. In addition to the above advantages, silicon carbide power devices can operate with lower specific on-resistance than conventional silicon power devices. Some of the advantages of using silicon carbide for forming power semiconductor devices are described in articles by K. Shenai, R. S. Scott, and coinventor B. J. Baliga, entitled Optimum Semiconductors for High-Power Electronics, IEEE Transactions on Electron Devices, Vol. 36, No. 9, pp. 1811-1823 (1989); and by coinventors M. Bhatnagar and B. J. Baliga entitled Analysis Of Silicon Carbide Power Device Performance, ISPSD '91, Abstr. 8.3, pp 176-180 (1991).
Many of the processes for forming microelectronic devices require the formation of P-N junctions in a semiconductor substrate. Conventional techniques for forming P-N junctions in semiconductor substrates include, for example, in-situ doping during the growth of semiconductor layers, implantation and diffusion of deposited dopant species, as well as other conventional techniques. For semiconductors such as silicon, it is possible to obtain P-N junctions well beneath the surface of the substrate using diffusion since diffusion rates for dopants in silicon are relatively high even at relatively low temperatures on the order of 1000-1200 degrees Centigrade. Accordingly, it is possible to obtain P-N junctions in silicon as deep as 1-3 microns. In silicon carbide, however, the diffusion coefficients of conventional P and N-type dopants are small in the temperature range of 1000-1200 degrees Centigrade. In fact, temperatures on the order of 1500 degrees Centigrade and higher are generally required for diffusion to occur at appreciable rates.
For example, in an invited paper by R. J. Trew, J. B. Yah and P. N. Mock, entitled The Potential of Diamond and SiC Electronic Devices for Microwave and Millimeter-Wave Power Applications, Proc. of the IEEE, Vol. 79, No. 5, pp. 598-620 (1991), temperatures on the order of 1900 degrees Centigrade were specified as being required for the diffusion of N or P-type dopants in silicon carbide. Unfortunately, this extreme range of temperatures is not compatible with the fabrication of integrated semiconductor devices having multiple layers of different conductivity type material. These temperatures are also considerably above the melting point of SiO.sub.2, a diffusion masking material having no commercially acceptable alternative for high temperature processing. Given these limitations, it is generally accepted that P-N junction formation arising from epitaxial growth or ion implantation with boron (B) or aluminum (Al) (p-type) or phosphorus (P) or nitrogen (N) (n-type) is most suitable for silicon carbide.
Unfortunately, although there has been a general acceptance of ion-implantation as a technique for forming P-N junctions in silicon carbide, problems including out-diffusion of dopant species, the precipitation of defect clusters, and the formation of electrically active line and point defects causing poor dopant ionization are encountered in the formation of lateral MESFETs and MOSFETs. For example, in an article by J. W. Bumgarner, H. S. Kong, H. J. Kim, J. W. Palmour, J. A. Edmond, J. T. Glass, and R. F. Davis, entitled Monocrystalline .beta.-SiC semiconductor Thin Films: Epitaxial Growth, Doping and FET Device Development, 1988 Proc. 38th Electronics Components Conf., pp. 342-349, solid phase epitaxial re-growth of the amorphous regions caused by the implantation of boron was achieved by annealing at 1600 degrees Centigrade for 300 seconds. However, defect clusters of precipitates and vacancy loops formed near the center of the amorphous regions within the implanted region. Subsequent annealing at 1800 degrees Centigrade for 300 seconds promoted virtually defect free regrowth, but SIMS analysis revealed almost complete out-diffusion of the implanted boron ions.
The implantation of P or N-type dopant species also results in the formation of an implant profile having a peak concentration below the implant surface. As well understood by those skilled in the art, the implant profile can generally be approximated by a Gaussian curve, or for greater accuracy, a four-moment Pearson-IV curve. However, even after annealing, which may cause diffusion away from the peak concentration region, a non-uniform doping profile in the N or P-type region is present. Accordingly, it is difficult to achieve a great degree of uniformity in regions formed by ion implantation.
Thus, notwithstanding the general acceptance of ion-implantation as a technique for forming regions of P and N-type conductivity in silicon carbide, problems still exist in obtaining substantially monocrystalline silicon carbide regions with uniform doping throughout.